Pectin is an important commodity in today's industry. For example, it can be used in the food industry as a thickening or gelling agent, such as in the preparation of jams.
Pectin is a structural polysaccharide commonly found in the form of protopectin in plant cell walls. The backbone of pectin comprises .alpha.-1-4 linked galacturonic acid residues which are interrupted with a small number of 1,2 linked .alpha.-L-rhamnose units. In addition, pectin comprises highly branched regions with an almost alternating rhamno-galacturonan chain. These highly branched regions also contain other sugar units (such as D-galactose, L-arabinose and xylose) attached by glycosidic linkages to the C3 or C4 atoms of the rhamnose units or the C2 or C3 atoms of the galacturonic acid units. The long chains of .alpha.-1-4 linked galacturonic acid residues are commonly referred to as "smooth" regions, whereas the highly branched regions are commonly referred to as the "hairy regions".
Some of the carboxyl groups of the galacturonic residues are esterified (e.g. the carboxyl groups are methylated). Typically esterification of the carboxyl groups occurs after polymerisation of the galacruronic acid residues. However, it is extremely rare for all of the carboxyl groups to be esterified (e.g. methylated). Usually, the degree of esterification will vary from 0-90%. If 50% or more of the carboxyl groups are esterified then the resultant pectin is referred to as a "high ester pectin" ("HE pectin" for short) or a "high methoxyl pectin". If less than 50% of the carboxyl groups are esterified then the resultant pectin is referred to as a "low ester pectin" ("LE pectin" for short) or a "low methoxyl pectin". If the pectin does not contain any--or only a few--esterified groups it is usually referred to as pectic acid.
The structure of the pectin, in particular the degree of estenfication (e.g. methylation), dictates many of the resultant physical and/or chemical properties of the pectin. For example, pectin gelation depends on the chemical nature of the pectin, especially the degree of esterification. In addition, however, pectin gelation also depends on the soluble-solids content, the pH and calcium ion concentration. With respect to the latter, it is believed that the calcium ions form complexes with free carboxyl groups, particularly those on a LE pectin.
Pectic enzymes are classified according to their mode of attack on the galacturonan part of the pectin molecule. A review of some pectic enzymes has been prepared by Pilnik and Voragen (Food Enzymology, Ed.: P. F. Fox; Elsevier; (1991); pp: 303-337). In particular, pectin methylesterases (EC 3.1.1.11), otherwise referred to as PMEs, de-esterify HE pectins to LE pectins or pectic acids. In contrast, and by way of example, pectin depolymerases split the glycosidic linkages between galacturonosyl methylester residues.
In more detail, PME activity produces free carboxyl groups and free methanol. The increase in free carboxyl groups can be easily monitored by automatic titration. In this regard, earlier studies have shown that some PMEs de-esterify pectins in a random manner, in the sense that they de-esterify any of the esterified (e.g. methylated) galacturonic acid residues on more than one of the pectin chains. Examples of PMEs that randomly de-esterify pectins may be obtained from fungal sources such as Aspergillus aculeatus (see WO 94/25575) and Aspergillus japonicus (Ishii et al J Food Sci 44 pp 611-14). Baron et al (Lebensm. Wiss. M-Technol 13 pp 330-333) apparently have isolated a fungal PME from Aspergillus niger. This fungal PME is reported to have a molecular weight of 39000 D, an isoelectric point of 3.9, an optimum pH of 4.5 and a K.sub.m value (mg/ml) of 3.
In contrast, some PMEs are known to deesterify pectins in a block-wise manner, in the sense that it is believed they attack pectins either at non-reducing ends or next to free carboxyl groups and then proceed along the pectin molecules by a single-chain mechanism, thereby creating blocks of unesterified galacturonic acid units which are very calcium sensitive. Examples of such enzymes that block-wise enzymatically de-esterify pectin are plant PMEs. Up to 12 isoforms of PME have been suggested to exist in citrus (Pilnik W. and Voragen A. G. J. (Food Enzymology (Ed.: P. F. Fox); Elsevier; (1991); pp: 303-337). These isoforms have different properties.
Versteeg et al (J Food Sci 45 pp 969-971) apparently have isolated a PME from orange. This plant PME is reported to occur in multiple isoforms of differing properties. Isoform I has a molecular weight of 36000 D, an isoelectric point of 10.0, an optimum pH of 7.6 and a K.sub.m value (mg/ml) of 0.083. Isoform II has a molecular weight of 36200 D, an isoelectric point of 11.0, an optimum pH of 8.8 and a K.sub.m value (mg/ml) of 0.0046. Isoform III (HMW-PE) has a molecular weight of 54000 D, an isoelectric point of 10.2, an optimum pH of 8 and a K.sub.m value (mg/ml) of 0.041. However, to date there has been very limited sequence data for such PMEs.
According to Pilnik and Voragen (ibid), PMEs may be found in a number of other higher plants, such as apple, apricot, avocado, banana, berries, lime, grapefruit, mandarin, cherries, currants, grapes, mango, papaya, passion fruit, peach, pear, plums, beans, carrots, cauliflower, cucumber, leek, onions, pea, potato, radish and tomato. However, likewise, to date there has been very limited sequence data for such PMES.
Random or blockwise distribution of free carboxyl groups can be distinguished by high performance ion exchange chromatography (Schols et al Food Hydrocolloids 1989 6 pp 115-121). These tests are often used to check for undesirable, residual PME activity in citrus juices after pasteurisation because residual PME can cause, what is called, "cloud loss" in orange juice in addition to a build up of methanol in the juice.
PMEs have important uses in industry. For example, they can be used in or as sequestering agents for calcium ions. In this regard, and according to Pilnik and Voragen (ibid), cattle feed can be prepared by adding a slurry of calcium hydroxide to citrus peels after juice extraction. After the addition, the high pH and the calcium ions activate any native PME in the peel causing rapid de-esterification of the pectin and calcium pectate coagulation occurs. Bound liquid phase is released and is easily pressed out so that only a fraction of the original water content needs to be removed by expensive thermal drying. The press liquor is then used as animal feed.
Pilnik and Voragen (ibid) list uses of endogenous PMEs which include their addition to fruit juices to reduce the viscosity of the juice if it contains too much pectin derived from the fruit, their addition as pectinase solutions to the gas bubbles in the albedo of citrus fruit that has been heated to a core temperature of 20 to 40.degree. C. in order to facilitate removal of peel and other membrane from intact juice segments (U.S. Pat. No. 4,284,651), and their use in protecting and improving the texture and firmness of several processed fruits and vegetables such as apple (Wiley & Lee 1970 Food Technol 24 1168-70), canned tomatoes (Hsu et al 1965 J Food Sci 30 pp 583-588) and potatoes (Bartolome & Hoff 1972 J Agric Food Chem 20 pp 262-266).
Glahn and Rolin (1994 Food Ingredients Europe, Conf Proceedings pp 252-256) report on the hypothetical application of the industrial "GENU pectin type YM-100" for interacting with sour milk beverages. No details are presented at all on how GENU pectin type YM-100 is prepared.
EP-A-0664300 discloses a chemical fractionation method for preparing calcium sensitive pectin. This calcium sensitive pectin is said to be advantageous for the food industry.
Thus, pectins and de-esterified pectins, in addition to PMEs, have an industrial importance. However, there is a continuing need to improve the known methods of stabilising proteins in an acidic environment but without adversely affecting the viscosity of that environment. In this regard, an adverse effect on the viscosity of the environment can compromise the overall appearance and/or texture and/or palatability and/or mouth feel of the resultant product.